May 1994
Immediately Dangerous to Life or Health Concentrations (IDLH)

CAS number: 109–87–5

NIOSH REL: 1,000 ppm (3,100 mg/m3) TWA

Current OSHA PEL: 1,000 ppm (3,100 mg/m3) TWA

1989 OSHA PEL: Same as current PEL

1993-1994 ACGIH TLV: 1,000 ppm (3,110 mg/m3) TWA

Description of substance: Colorless liquid with a chloroform-like odor.

LEL:. . . 2.2% (10% LEL, 2,200 ppm)

Original (SCP) IDLH: 15,000 ppm [LEL]

Basis for original (SCP) IDLH: Patty [1963] reported that Weaver et al. [1951] noted only minor irritation in 50 mice that received fifteen 7-hour exposures to 11,000 ppm and that 6 of 50 mice died after fifteen 7-hour exposures to 18,000 ppm. Because the data indicate that acutely toxic effects occur only above the lower explosive limit (LEL) of 16,000 ppm, the IDLH is based on the LEL rounded down to 15,000 ppm. This is the concentration above which only the “most protective” respirators are permitted.

Short-term exposure guidelines: None developed


Lethal concentration data:

Species Reference LC50




Time Adjusted 0.5-hr


Derived value
Mouse Marhold 1986 18,000 ----- 7 hr 43,291 ppm (2.4) 4,329 ppm
Rat NPIRI 1974 15,000 ----- ? ? ?
Mouse Weaver et al. 1951 18,354 ----- 7 hr 44,050 ppm (2.4) 4,405 ppm

Lethal dose data:

Species Reference Route LD50




Adjusted LD Derived value
Rabbit Knoefel et al. 1932 oral 5,708 ----- 12,644 ppm 1,264 ppm

Other animal data: In one study, the "no effect" levels were determined to be 2,810 and 8,450 ppm in rats and guinea pigs, respectively [Price et al. 1978]. Fifteen 7-hour exposures to 11,000 ppm caused only minor irritation in 50 mice [Weaver et al. 1951].

Human data: Methylal has been used as an anesthetic in surgery [ACGIH 1991].

Revised IDLH: 2,200 ppm [LEL]

Basis for revised IDLH: Based on health considerations and acute inhalation toxicity data in humans [ACGIH 1991] and animals [Price et al. 1978; Weaver et al. 1951], a value of about 10,000 ppm would have been appropriate for methylal. However, the revised IDLH for methylal is 2,200 ppm based strictly on safety considerations (i.e., being 10% of the lower explosive limit of 2.2%).


1. ACGIH [1991]. Methylal. In: Documentation of the threshold limit values and biological exposure indices. 6th ed. Cincinnati, OH: American Conference of Governmental Industrial Hygienists, pp. 937-938.

2. Knoefel PK, Lonergan L, Leake CD [1932]. Biochemorphic aspects of paraldehyde and certain acetals. Proc Soc Exp Biol Med 29:730-732.

3. Marhold J [1986]. Prehled Prumyslove Toxikologie, Organicke Latky. Prague, Czechoslovakia: Avicenum, p. 259 (in Czechoslovakian).

4. NPIRI [1974]. Raw materials data handbook, physical and chemical properties, fire hazard and health hazard data. Vol. 1. Organic solvents. Bethlehem, PA: National Printing Ink Research Institute, p. 73.

5. Patty FA, ed. [1963]. Industrial hygiene and toxicology. 2nd rev. ed. Vol. II. Toxicology. New York, NY: Interscience Publishers, Inc., pp. 1984-1987.

6. Price NH, Allen SD, Daniels AU, Yates WG [1978]. Toxicity data for establishing “immediately dangerous to life or health” (IDLH) values. Salt Lake City, UT: University of Utah Research Institute, UBTL Division, Report No. TR 1510-005, Contract No. CDC-210-76-0143.

7. Weaver FL, Hough AR, Highman B, Fairhall LT [1951]. The toxicity of methylal. Br J Ind Med 8:279-283.

Page last reviewed: December 4, 2014